1. Field of the Invention
The present invention relates to a digital information reproducing apparatus for reading out information from a digital signal obtained through a band-limited transmission system, and more particularly to an apparatus capable of controlling the threshold level of a waveform shaper so as to correctly read the digital signal.
2. Description of the Prior Art
For the purpose of transmitting digital signals at a high transfer rate or recording them at a high density, there is often employed recently a technique of digital modulation which transmits or records digital signals within a base band. In such a signal transmission using a base band, it is generally necessary to transmit even the lowest frequency range including a DC component and is further necessary to shape the rounded waveform which is deformed in the transmission line to a proper shape at a predetermined threshold level. However, there exists a problem in that an optimal threshold level drifts because some low frequency noise is prone to mix into the input signal due to temperature drift in a circuit or nonuniformity of a recording medium used. For solving such a problem, there is known a conventional method which controls the threshold level to an optimal value for elimination of any harmful influence that results from the drift of the threshold level caused by low frequency noise.
With regard to a channel code signal processed by a DC-free modulation system according to which the DC component of the signal to be transmitted is not modulated, it is possible to attain an optimal threshold level by controlling the threshold level in such a manner that the DC component of the waveform-shaped signal obtained from the transmitted input signal becomes a predetermined value. An exemplary waveform shaper for controlling the threshold level as mentioned above is disclosed in Japanese Laid-Open Patent Application No. 57 (1982)-78611.
In such a conventional waveform shaper, however, controlled retention of the threshold level at an optimal value is not achievable if the input is not a channel code signal processed by the DC-free modulation system. For example, in the so-called 2-7 code modulation employed frequently in the recent magnetic storage systems , variations occur in the DC component of the modulated channel code signal, and consequently it is impossible to attain optimal control of the threshold level by the above-noted method.
Also with respect to the channel code signal modulated by system other than a DC-free modulation system, it is still possible to accomplish optimal control of the threshold level by detecting the threshold level error with reference to a read clock extracted from the channel code signal. An exemplary-waveform shaper for controlling the threshold level in this manner is disclosed in Japanese Laid-Open Patent Application No. 59 (1984)-113529, wherein the threshold level is controlled in accordance with a difference obtained from the phase difference between the rising edge of the shaped input signal and the read clock and the phase difference between the falling edge and the read clock.
However, in the method which detects the threshold level error with reference to the read clock as mentioned above, if there occurs a variation in the threshold level in such a manner that the pulse duration changes by a length comparable to the read clock period, the signal with such a pulse duration change may be erroneously recognized as the original signal. In such a case, a spurious lock state occurs where the threshold level is set to an improper value.